The present disclosure is directed to an pulse height conversion or analog-to-digital circuit and particularly one useful in digitizing pulse height signals scattered over a wide energy range resulting from pulse neutron irradiation of formations adjacent to a cased well. A logging sonde having a pulsed neutron source is lowered on a cable into a well to provide periodic bursts of neutrons into the immediate formations. The response of the formations is measured by observation of a scintillation counter or photomultiplier tube (PMT). The formation response has the form of a number of close nuclear events of irregular spacing, but typically is an avalanche of pulses where pulse energy level varies over a relatively wide range. This provides an output signal from a typical PMT which is a plurality of pulses of variant height. The pulses occur so rapidly that it is difficult to obtain a count of the pulses as well as pulse height measurements. This measurement must be accomplished in a logging tool which is lowered in downhole circumstances, exposed to extremely high temperatures and subjected to shock loading as the sonde is retrieved from the well. The present apparatus is a pulse height measuring system which digitizes the variable energy nuclear events, provides a digitized measure thereof, and which operates at sufficient speed that the consequential burst of nuclear events can be measured.